Many electric rail transit system cars in existence today are provided with fuse assemblies which are constructed as part of the wheel and drive assemblies. These fuse assemblies, which may be rated at, e.g., 700 amperes, have an elongated housing commonly made of plastic with a simple ribbon fuse connected between electrical terminals at opposite ends thereof. The housing has a base which is secured to part of the current collector or vehicle structure but it has no cover. The fuse element is thus exposed to the outside and, of course, to whatever dirt or weather conditions might exist.
The housing also usually has elements of a magnetic field shaping device at opposite sides of the middle of the housing, on either side of a narrowed portion of the ribbon fuse element, to help quench any arc which might develop at that location.
Because of the heat and arcs, etc. which can be developed in a fuse of this type, it is not practical to put a cover on the housing. Accordingly, it is simply left open and presents a potential shock hazard as well, even though it is normally mounted on the "third rail" side of the train car and is therefore accessible only to trained maintenance personnel who should know enough to keep away from the fuse. A further hazard is thermal and mechanical damage to the vehicle from unlimited currents passed by a ribbon fuse.
Another reason for leaving the housing open is so that the condition of the fuse element can be visually checked. A glance is all that is necessary to see if the element has blown.